1. Introduction
Photovoltaic (PV) arrays, wind turbines, and fuel cells are all examples of Distributed Generators (DGs) that can be integrated into an existing Distribution Network (DN) to save energy costs, improve reliability, and satisfy environmental regulations. Furthermore, utilities benefit from higher power quality with such DG systems in place. However, DGs cause a change in the level and characteristics of the fault current and bidirectional power flow [
1,
2,
3]. As a result, the conventional method of protecting DN with DGs based on the use of Overcurrent Relays (OCRs) is becoming more challenging. In addition, when designing a protective system to deal with varying fault levels and different grid operation modes (with and without DG), the DN’s ability to operate in these modes is an important concept to take into account [
4]. Redesigning or replacing the protection system in a DN due to the integration of the DGs can be costly and technically challenging. By increasing the capacity of DG penetration in DN, the OCR settings must be modified to cope with the changing power flow and fault. Adaptive protection systems in radial DNs often adjust the relay settings according to the role of the communication system. However, in many DNs, establishing a communication infrastructure for power protection systems may be an expensive choice. The recent and main OCR coordination approaches for DN with DG are divided into six categories, which are shown in
Figure 1, as follows [
4,
5]:
Developing a new objective function to address the OCR coordination problem.
Applying and developing a dual-setting approach in OCR.
Designing and developing a non-standard characteristic for OCR.
Using a new optimisation algorithm to solve the complex OCR coordination problem for a DN with DG.
Applying and developing new constraints to the objective function.
Designing and developing a hybrid tripping characteristic for OCR.
The primary purpose of all of the aforementioned approaches is to obtain the appropriate setting for protection schemes to maintain the reliability of protection scheme performance for interconnected DNs with DGs. Therefore, this research presents a new non-standard current–voltage characteristic for programmable OCRs as a hybrid tripping characteristic approach. The magnitudes of the faulty phase voltage and current are used by the suggested method to calculate the relay’s operation time by employing a new optimisation method. Plus, it can safeguard DNs with extensive penetration of DGs in the grid-connected mode of DN operation, and it does not rely on any sort of communication infrastructure. Academics and industry professionals have focused significantly on the problem of DN protection due to the stochastic behaviour of DGs. Several approaches have been proposed to provide an adequate method of protecting the microgrid in both modes (with and without DGs). Some of them concentrate on communication channel protection [
6,
7]. Slemaisar-doo et al. [
8] suggested a differential protection approach employing a non-nominal frequency-current during a microgrid fault that is superior to traditional overcurrent protection in detecting the microgrid fault. Aghdam et al. [
9] proposed a differential protection method based on variable tripping times, and a multiagent protection scheme was designed to improve the coordination of adjacent relays. Communication-based strategies are realistic microgrid protection options. However, the reliability of this type of protection highly depends on the communication facilities and performance; it is also not an economically viable solution. In addition [
6,
10], these schemes are affected by a communication failure, imbalanced loads, and transients’ events during the connection and disconnection of DGs. These days, it is common practice to employ programmable relays (microprocessor relays) to apply non-standard characteristics. The literature proposed several concepts, including a logarithm characteristic for OCRs [
5,
10], a combination of standard characteristics and a non-standard term based on voltage [
5], and a standard characteristic under new constants [
5,
10] for DNs with DG to reduce the total amount of time spent operating the OCR. However, these approaches are utilised in radial DNs with DG, and all of these techniques require a communication infrastructure. Therefore, the purpose of this study is to introduce a new hybrid optimal coordination strategy that does not require a communication link between the OCRs. This is intended to reduce the demand for communications infrastructure while improving the coordination approach of the OCR. Furthermore, the suggested method will reduce computing costs and the necessity to access voluminous PV and network data.
Because of phase OCR’s inapplicability for handling the complexity of DN-integrated PV systems, the voltage term is being explored as a potential term for solving the OCRs coordination issue [
11]. In [
12], a voltage-restricted overcurrent relay is presented using phase voltage and current to set the necessary threshold. Nevertheless, the PV plant’s control method may cause phase currents to be larger in a healthy phase than in a faulty phase, leading to the relay in miscoordination events. Few studies have looked into the use of voltage terms in the OCRs coordination problem [
13]. The use of voltage-restricted OCRs coordination schemes for network protection was discussed, for example, in [
14]. However, no voltage limitation was presented with the OCR algorithm or result [
14]. The voltage–current–time inverse model presented by Singh et al. [
15] is based on variations in currents and voltages during fault events. The suggested OCR coordination model improves operating time and maintains protection coordination for power networks with PV systems without considering different modes of the grid. El-Sayed et al. [
16] introduced a current–voltage scheme for directional OCR based on measuring harmonic currents and voltages at the relay position to guarantee optimal protection coordination for the islanded microgrid without taking into account bidirectional power flow and grid-connected operation mode. Protection schemes that use conventional inverse time–current characteristics are becoming increasingly unsuitable as the penetration of DGs in the DN rises. Authors [
17,
18] presented a voltage-based protection scheme to minimise the operating time of the relay compared to the traditional inverse time current scheme. Another study [
19] proposes a strategy for protecting the DN using the superconducting fault limiter based on the voltage parameter. There is a limited number of research that used the non-standard logarithmic function for developing a current–voltage protection coordination scheme for DN with DGs under different grid operation modes and fault scenarios.
The foregoing challenges and evidence point to the necessity of having a flexible protection mechanism for more dynamic power networks with DG in the future. OCR protection schemes must be compatible with dynamic power systems in terms of their ability to overcome and accommodate these emerging features, raising questions about the appropriateness of standard characteristics of OCR protection schemes, stability, and protection selectivity for DN with DG. In this paper, we introduce a novel hybrid tripping scheme based on non-standard current–voltage characteristics for fast response OCR prevention in DNs with PV farm systems without using a communication protocol. The following are some of the study’s contributions that aim to bridge this research area gap:
To enhance the performance of the protection system, a new non-standard logarithmic and hybrid tripping coordination scheme based on current–voltage characteristics is established for DN with DGs. In this article, a significant reduction in total operational time is achieved, with no instances of miscoordination compared to typical characteristics of the OCR scheme. This work compares the proposed new hybrid tripping OCR scheme (HOC) with the common inverse time–current characteristic (SIC) and the time–current–voltage characteristic (CVC) from the literature [
16,
20].
In the literature [
20], the use of modern optimisation techniques in solving protection problems, such as the particle swarm algorithm [
21,
22] assists in achieving the optimal OCR settings. To solve the OCR coordination problem based on the non-standard current–voltage characteristic and reduce the total operational time of OCRs, a new optimisation technique, the Vibrating Particles System (VPS) approach, has been designed and employed.
Since the proposed hybrid optimal coordination scheme in this work only uses locally obtained measurements, no medium of communication between the OCRs is necessary. This eliminates the demand for communications infrastructure and reduces computational costs and the requirement for access to extensive PV and network data.
The sensitivity and selectivity of the proposed hybrid optimal coordination scheme have been investigated for DN (benchmark power network, CIGRE) with DGs under different fault scenarios and operation modes. This aims to provide network operators with a preliminary indicator regarding the potential impact of DGs on the fault contribution and relay setting.
The following sections are ordered as follows: The optimum OCRs coordination problem formulation is introduced in
Section 2. In addition, the proposed hybrid tripping scheme is developed. The results and comparison are discussed in
Section 3. In
Section 4, the summary and conclusion of this work are presented.
4. Conclusions
The goal of this work was to implement a rapid response hybrid tripping scheme for the OCR protection system in modern power network architecture (with and without PVs). The suggested HOC scheme controls and minimises the total operational time for all OCRs by utilizing a new current–voltage characteristic. The optimal OCRs coordination problem under grid constraints was established using the HOC method. The suggested HOC was formulated and solved to provide optimal OCR settings under various fault conditions and different network topologies. The VPS and PSO algorithms were employed to fine-tune and resolve the optimal configuration of all OCRs. The consequences of the suggested HOC approach were not only the fulfilment of the coordination assignment but also a significant reduction in the overall operating time compared to the common SIC and CVC. For example, the overall operational times for the power grid connected to PVs were 11.86, 11.14, and 7.68 s for SIC, CVC, and HOC approaches, respectively. In addition, the VPS and PSO optimisation methods were evaluated to identify a solution with a rapid trip time. The high and complex protection challenges will, in the future, require using machine learning, artificial intelligence, and different optimisation algorithms to minimise the tripping time and improve the protection selectivity performance.